The Perimeter of a shape is defined as the distance around the shape. Since we usually discuss the perimeter of polygons (closed plane figures whose sides are straight line segment), we are able to calculate perimeter by just adding up the lengths of each of the sides. When we talk about the perimeter of a circle, we call it by the special name of a circumference. Since we don’t have straight sides to add up for the circumference (perimeter) of a circle, we have a formula for calculating this.
Equilateral Triangle 
Isosceles Triangle 
Scalene Triangle 
P = 3.I 
P = 2.I + b 
P = a + b + c 

Perimeter of a Triangle formula is given as P
“P” is defined as Perimeter hence, P = a + b + c
This formula implies to find the perimeter of a triangle, you add the lengths of all of its 3 sides together.
Check the triangle and regulate the lengths of the three sides.
Example below,Side a length = 10, and side b = 10, and side c = 10.
This is called an equilateral triangle since all three sides are equal in length. Perimeter formula for any triangle is always the same.
To find the perimeter add the three side lengths together
In this example, 10 + 10 + 10 = 30. Therefore, P = 30.
For example, for another triangle where a = 5, b = 2, and c=3, the perimeter would be: P = 5 + 2 + 3, or 10
P = 5 + 2 + 3, or 10
Always include units in the final answer. If the sides of the triangle are measured in centimetres then finals answer should also be in centimeters.
In this example, The sides of the length are each 10 cm, so the correct value for Perimeter is 30 cm.
Examples:
Find the perimeter of a parallelogram whose sides are 5 cm, 4 cm and 2 cm?
Solution
Given,
a = 5 cm
b = 4 cm
c = 2 cm
Perimeter of Triangle = a + b + c
5 + 4 + 2 = 20 cm